Conventionally, to drive a motor of this type, using pulse width modulation, a voltage of pseudo-sinusoidal wave is generated and applied, and the momentary value of obtained sinusoidal current, the applied voltage, and the parameter model of the motor are used, and by re-calculation from the parameter model, the current value of the model output and the applied voltage of the model output are obtained, and by feedback calculation so as to modulate the estimated phase information until the error from the model output becomes zero, and thereby the magnetic pole position is estimated (see, for example, non-patent document 1).
In a simpler method, specific voltage are applied in a partial period only, for example, at an electrical angle of 120 degrees, the voltage of the terminal is detected when voltage is not applied, and the magnetic pole position is calculated (see, for example, non-patent document 2). FIG. 7 shows a conventional motor drive control device according to non-patent document 2. As shown in FIG. 7, in the conventional motor drive control device, an alternating-current power supply 4 is rectified by a rectifying circuit 3, and is smoothed by a capacitor 8, and a direct current is obtained. The direct current is supplied into a three-phase bridge circuit 2, and is further applied into a control circuit 95, and a semiconductor switch in the three-phase bridge circuit 2 is turned on or off, and thereby a dummy three-phase alternating current is applied to a motor 1. This dummy three-phase alternating current is an alternating current cyclically repeating positive direction feeding, feeding off, negative direction feeding, and feeding off. To detect the magnetic pole positions of the motor 1, the voltages in non-feeding period is filtered by filters (97u, 97v, 97w), and compared in comparators (96u, 96v, 96w) with the reference voltage or the average of three voltages, and the magnetic pole positions are directly detected from the induced voltage waveform.    Non-patent document 1: Collected papers of Society of Electric Engineers, D, vol. 117, No. 1, 1997, pp. 98-104.    Non-patent document 2: Eiichi Ono, “Introduction to power electronics” (3rd ed.), Ohm-Sha, Aug. 20, 1997, pp. 242-244